x(x-0.2)=640

Solution for x(x-0.2)=640 equation:

x(x-0.2)=640

We move all terms to the left:

x(x-0.2)-(640)=0

We multiply parentheses

x^2-0.2x-640=0

a = 1; b = -0.2; c = -640;
Δ = b2-4ac
Δ = -0.22-4·1·(-640)
Δ = 2560.04
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-0.2)-\sqrt{2560.04}}{2*1}=\frac{0.2-\sqrt{2560.04}}{2}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-0.2)+\sqrt{2560.04}}{2*1}=\frac{0.2+\sqrt{2560.04}}{2}
$